Assign Office 365 global administrator role to a user account

In the Office 365 connection wizard to begin the migration process if you want to connect using global admin user then select “ Connect using global admin user” and select the required mail items and click “ next” to move into further step.

Now the login wizard will open in that you need to login with your organizational login credentials to allow you to continue working with migration process

After successful login, below message box will be displayed. If you choose “yes” it will display previously stored mailboxes

If you choose “No” next wizard will open to display list of mailboxes select one of them

This option lists all the mailboxes for it failed to load then you can easily load using .CSV file.

In this option you have to do the following steps to load mailboxes

This option will display all the mailboxes which are stored previously.

Using Global Admin Account it lets you to use the single Global Admin account to migrate any Exchange server mailboxes no need to login for each and every users for effective email migration.
If you are using Global Admin then you need to use Global Admin account for login and you must set impersonation rights to the Global Admin account otherwise application does not allow you to access source mailbox and to take everything as planned by the application.